(1) Field of the Invention
This invention relates to a method and apparatus for vehicle navigation. More specifically the invention relates to a method and apparatus for obtaining navigational information in an underwater vehicle using a single vector sensor on the vehicle and a single transducer positioned in the environment.
(2) Description of the Prior Art
Obtaining precise navigational coordinates is a longstanding problem in underwater vehicles. Conventional global positioning systems (GPS) rely on radio wave reception from satellites to establish navigational coordinates, but bodies of water block all but the lowest radio frequencies. Accordingly, underwater vehicles must either surface an antenna or find some other method for obtaining navigational information.
Acoustic pressure sensing hydrophones are commonly used in manned and unmanned underwater vehicle applications. These non-directional sensors receive sound equally from all directions. To obtain directivity, the hydrophones are configured in an array consisting of many elements. The array assembly feeds acoustic information to signal processors for creating beam patterns that have directionality. The directionality of the beam is controlled by the relative size of the array to an acoustic wavelength. Thus, higher frequency acoustic wavelengths are used to increase the directionality. The useful frequency band of the array in a small vehicle, such as an unmanned vehicle, is limited to high frequencies; however, these high frequencies attenuate over a short distance thereby reducing the useful navigation capabilities.
Acoustic vector sensors have been developed which measure non-directional acoustic pressure and vector acoustic velocity components of an acoustic signal. Three vector acoustic velocity components are measured orthogonally. These vector components can be electronically steered to provide increased or decreased sensitivity at a given location.
One current underwater navigational system uses a single omnidirectional pressure sensor on a vehicle and four transponders. Utilizing technology known as hyperbolic multilateration, one can get an x, y, z position fix from the range to each transponder. This technique is similar to that used in the Global Positioning System (GPS). However, the system is limited to a relatively high frequency (7 kHz to 40 kHz) which reduces the operational range from the transducers.
Another current navigational system uses a high frequency array of sensors on the vehicle. Two transponders must be positioned in the environment to get an x, y, and z position fix. Triangulation is used to obtain directional information to the transponders. The frequency used is greater than 20 kHz, limiting the range. This system is expensive because of the needed array.
Another known navigational system utilizes a GPS buoy which acts as a relay to communicate to underwater vehicles by using an acoustic communication link. This system requires deployment of the GPS buoy within communication range of the underwater vehicle. The GPS buoy is further limited by the existing surface sea state.